libdwelf: Add dwelf_elf_e_machine_string and use it in readelf.

libdwelf: Add dwelf_elf_e_machine_string and use it in readelf.

To print eh human readable description of the ELF e_machine header field
we used the ebl name. But this is not set for most EM constants. Introduce
a new function dwelf_elf_e_machine_string that does work for all known
EM values. Use that in eu-readelf to print a string representation of the
e_machine value.

Since this was the only usage of ebl->name, remove that from struct ebl.

Also add a testcase that makes sure dwelf_elf_e_machine_string works for
all EM values in the libelf/elf.h header so we will immediately notice
when a new value appears.

Signed-off-by: Mark Wielaard <mark@klomp.org>